This invention relates, in general, to Doppler compensation, and more specifically, to calibration of Doppler within a narrow bandwidth.
Satellites are becoming important links for communication between stations at different locations throughout the world, particularly for mobile communication units. Such mobile communication units, particularly hand held communication units, inherently operate on low power. To operate with low power, the hand-held communication units must be designed for narrow band signals for efficiency. Typical applications for narrow band designs include hand-held units which must communicate with low orbiting satellites (e.g. approximately 400 n. mi.).
The Doppler effect on signals transmitted between a mobile communication unit and a satellite becomes distorted, particularly for low-earth-orbiting satellites. The distortion caused by Doppler must be removed for the information to be extracted when the communication channel is narrow (e.g. 3 Khz). For instance, a signal transmitted to earth from a low orbiting satellite would have a frequency variation due to Doppler of .+-.35 Khz at a center frequency of 1.5 Ghz over a period of 10 to 15 minutes. Voice channels of 3 Khz would require guard band of approximately 12 channel widths between voice/data channels to avoid interference between channels. When the channel width is even smaller, as with pagers having a band width of 300 Hz, the guard band increases to over 200 channel widths.
The problem of guard band width may be overcome by transmitting a pilot signal or control signal in a single dedicated channel. The control signal must be sufficiently separated from the voice/data signals that the voice/data signals do not interfere with the control signal. Terrestrial receivers search for the control signal, and use the control signal to direct data traffic for the other channels. By separating the control signal from all other signals, the Doppler effect on the control signal can be determined and applied to the voice/data channels. This avoids tracking Doppler on the voice/data channels.
Continuous tracking of the control signal to obtain Doppler requires a continuous supply of power. For low-power hand-held units, such as pagers and hand-held cellular telephones, continuous tracking of Doppler is inefficient and undesirable.